Gas is moving nearly at light-speed as the space it occupies is dragged around.

NGC 1365, the Great Barred Spiral Galaxy, in visible light and X-rays (inset). Astronomers determined the strong X-ray emissions from the black hole at the center of this galaxy are due in part to reflection off gas moving nearly at the speed of light.

Optical: SSRO-South, X-ray: NASA/CXC/CfA/INAF/Risaliti

The space near black holes is one of the most extreme environments in the Universe. The bodies' strong gravity and rotation combine to create rapidly spinning disks of matter that can emit huge amounts of light at very high energies. However, the exact mechanism by which this light is produced is uncertain, largely because high-resolution observations of black holes are hard to do. Despite their outsized influence, black holes are physically small: even a black hole a billion times the mass of the Sun occupies less volume than the Solar System.

A new X-ray observation of the region surrounding the supermassive black hole in the Great Barred Spiral Galaxy may have answered one of the big questions. G. Risaliti and colleagues found the distinct signature of X-rays reflecting off gas orbiting the black hole at nearly the speed of light. The detailed information the astronomers gleaned allowed them to rule out some explanations for the bright X-ray emission, bringing us closer to an understanding of the extreme environment near these gravitational engines.

Despite the stereotype of black holes "sucking" matter in, they attract it via gravity. That means stars, gas, and other things can fall into orbits around black holes, which may be stable for long periods of time. Gas often forms accretion disks and jets that release huge amounts of energy in the form of light. This energy can include X-ray emissions. So despite their name, black holes can be very luminous objects.

Nearer the boundary of a rotating black hole—its event horizon—the strength of gravity is such that the space matter occupies can be also dragged around the black hole. This effect is called "frame dragging," and is predicted by Einstein's general theory of relativity. The region in which frame dragging becomes significant, however, is very close to the black hole's event horizon, which is relatively small, especially when imaged from Earth. As a result, astronomers could not be sure whether ordinary orbital effects or relativistic frame-dragging is more important for producing the intense X-ray emissions.

Astronomers paid particularly close attention to the supermassive black hole at the center of the Great Barred Spiral Galaxy (also known by its catalog number NGC 1365) when a cloud of gas momentarily eclipsed it. That rare event allowed them to get a good size estimate for the accretion disk that surrounds the black hole. The current study followed up by monitoring fluctuations in the X-ray emissions, using the orbiting XMM-Newton and NuSTAR X-ray telescopes.

In particular, the researchers looked at emission from neutral and partly ionized iron atoms in the gas. Prior observations showed that the emission lines were broadened, which can be caused by several different phenomena. Researchers considered two primary hypotheses: absorption by other gas along the line of sight between the black hole and us, or very fast motion of the gas itself.

The new data strongly supported the latter option. In this scheme, the observed X-ray light reflected off the inner edge of the accretion disk, where the gas is moving at very close to the speed of light. According to the models, this scattering occured well within the frame-dragging region near the black hole. The inner edge of the accretion disk may be close to or at the minimum stable distance from the black hole. Closer than that distance, and matter can no longer orbit in a circular path—it will tend to spiral in.

The authors argued that any explanation of the X-ray emission that fails to account for the general-relativistic effects just won't work. Previous observations estimated that the black hole in the Great Barred Spiral Galaxy is spinning nearly as fast as possible; whether other black holes will have similar properties remains an open question.

Promoted Comments

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

Same way the electron, as a point particle, has spin. So long as you're massive, nothing prevents your wave function from carrying angular momentum, and when a black hole has angular momentum, space is dragged along for the ride. Actually, any rotating mass does this (it's been measured around the Earth) but it's most extreme around a black hole.

very interesting article. I always thought that the energy emitted from black hole relatively small and they couldn't be detected easily. I hope they will be able to crack the mystery as to what happens to matter and space when it enters the black hole and how it gets reshaped

even a black hole a billion times the mass of the Sun occupies less volume than the Solar System.

Umm, given that the solar system is roughly 2,000 sun diameters, and thus constitutes roughly 8,000,000,000 billion times the volume of the sun, I certainly would hope so. Otherwise it would be less dense, on the whole, than the sun. How it works out is that a 10^9 sun-mass blackhole has a Schwarzschild diameter, that is distance from event horizon on one side to the other, that is nor more than 1/5 the size of the solar system.

very interesting article. I always thought that the energy emitted from black hole relatively small and they couldn’t be detected easily.

Still true, it is still an indirect observation they are talking about. It is the mess around the black hole that is reflecting and generating EM, assumed to be gaseous matter that the black hole is in the process of accumulating.

For black holes that are not accumulating large amounts of matter like this we have to rely on gravity more directly such as gravimetric lensing, which is tricker because you need a suitable distant directly observable object behind it relative to our position.

Previous observations estimated that the black hole in the Great Barred Spiral Galaxy is spinning nearly as fast as possible; whether other black holes will have similar properties remains an open question.

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

Umm, given that the solar system is roughly 2,000 sun diameters, and thus constitutes roughly 8,000,000,000 billion times the volume of the sun, I certainly would hope so. Otherwise it would be less dense, on the whole, than the sun. How it works out is that a 10^9 sun-mass blackhole has a Schwarzschild diameter, that is distance from event horizon on one side to the other, that is nor more than 1/5 the size of the solar system.

A black hole of 10^9 solar masses is much less dense than the sun if you use the Schwarzschild radius to describe the volume of the black hole.

~18 kg/m^3 for the black hole vs 1408 kg/m^3 for the sun.

Whereas if you had a black hole of 10^-9 solar masses, then that would be ridiculously more dense than the sun at 1.8*10^37 kg/m^3

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

A black hole rotating so fast that its angular momentum (multiplied by the fundamental constants necessary to convert it to a distance) is larger than its mass squared (converted to a distance in a similar way) has no event horizon. We call this a naked singularity, and while it doesn't seem like GR itself allows a proof that naked singularities don't exist, simulations suggest and empirical evidence supports the idea that it's impossible for such a black hole to form. Disallowing naked singularities limits the rate at which black holes of a given mass can spin. Most black holes are right up against this limit, as a result of collapsing from much larger moderately spinning objects to their tiny sizes.

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

A black hole rotating so fast that its angular momentum (multiplied by the fundamental constants necessary to convert it to a distance) is larger than its mass squared (converted to a distance in a similar way) has no event horizon. We call this a naked singularity, and while it doesn't seem like GR itself allows a proof that naked singularities don't exist, simulations suggest and empirical evidence supports the idea that it's impossible for such a black hole to form. Disallowing naked singularities limits the rate at which black holes of a given mass can spin. Most black holes are right up against this limit, as a result of collapsing from much larger moderately spinning objects to their tiny sizes.

Even allowing naked singularities, no point on the event horizon would be able to exceed the speed of light, so depending on the radius, you could determine a maximum spin rate.

The event horizon isn't a physical object. The speed of light limit is local, so the space there can move as fast as it likes, dragging matter around from the perspective of an external observer at as large an apparent speed as it wants, so long as no matter on the event horizon locally exceeds c. Except for the angular momentum limit.

The event horizon isn't a physical object. The speed of light limit is local, so the space there can move as fast as it likes, dragging matter around from the perspective of an external observer at as large an apparent speed as it wants, so long as no matter on the event horizon locally exceeds c. Except for the angular momentum limit.

I've not studied Relativity in too much depth, but this gives me a context to understand something I've always had a problem with re: the idea that the speed of light is a hard limit. Thanks!

That is cool. From a non-physicist, would it be possible to utilize some of that speed like we would a gravitational slingshot?

The boost in speed from a gravitational sling shot actually comes from the velocity of the gravitational source, not the force of gravity itself. Kind of like how if you're in a car and throw a ball, the ball's full velocity is a combination of the car's velocity and your throwing velocity. So, yes, you could, if the black hole is moving, but not really any differently than you would get from a planet going the same way.

Previous observations estimated that the black hole in the Great Barred Spiral Galaxy is spinning nearly as fast as possible; whether other black holes will have similar properties remains an open question.

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

I wondered the same thing, and after googling thoroughly, couldn't find anything anywhere. All I could find was that the event horizon was moving at "nearly" or "85%" of the speed of light.

But why no rotations-per-minute (or similar) figure for this well-characterized black hole? It's a figure commonly given for neutron stars, for example.

The boost in speed from a gravitational sling shot actually comes from the velocity of the gravitational source, not the force of gravity itself. Kind of like how if you're in a car and throw a ball, the ball's full velocity is a combination of the car's velocity and your throwing velocity. So, yes, you could, if the black hole is moving, but not really any differently than you would get from a planet going the same way.

This is true, but black holes, because of frame dragging and their extreme rotational speeds, allow for a better mechanism. You can steal rotational energy from the rotating hole and accelerate to relativistic speeds for "free".

Previous observations estimated that the black hole in the Great Barred Spiral Galaxy is spinning nearly as fast as possible; whether other black holes will have similar properties remains an open question.

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

I wondered the same thing, and after googling thoroughly, couldn't find anything anywhere. All I could find was that the event horizon was moving at "nearly" or "85%" of the speed of light.

But why no rotations-per-minute (or similar) figure for this well-characterized black hole? It's a figure commonly given for neutron stars, for example.

You ask good questions and we have answers for you! The question was addressed somewhat generally here:

So let me run you through the basics. A black hole is entropic death. It is a state of maximum entropy, so don't expect to pull it apart or get anything back out - physics will prevent it. Ostensibly, you could spin it SO fast that its gravity could no longer hold stuff on the equator inside the event horizon. Obviously there must be a physical limit to how fast it can spin. Not only spin, but charge of the black hole could cause stuff to be ejected. Imagine that the black hole has a very large negative or positive charge. On Earth, if something has too strong of a charge it will discharge into the atmosphere, as you know. The same can happen in space if you have a voltage too ridiculously high. Like charges repel like, so if the black hole has a strong enough charge, it would eject matter, and we can not allow this. Another important details is that Coulomb's constant is much greater than the gravitational constant, so compared to the total mass, it wouldn't even that that much (relative) charge to do this.

So, we know that the black hole can't spin too fast or have too much of an electrical charge. What if we try to force it? It will force things right back. Basically, if the black hole spins too fast, then it won't accept any more spin (I say "spin" instead of angular momentum here). So if you take a wheel, spin it, and throw it into a black hole it won't take it. What will happen? Isn't that the exciting part!? No actually, physicists have a pretty good handle on this. It will just throw stuff away that gets near to it.

On this point, yes, the rotational energy of a black hole could, in fact, be harvested with relative ease, like other people have asked. It's entirely thinkable that a black hole could be used as a slingshot of sorts. If it is rotating close to the theoretical maximum, it will give up that extra rotation without much convincing needed.

The event horizon isn't a physical object. The speed of light limit is local, so the space there can move as fast as it likes, dragging matter around from the perspective of an external observer at as large an apparent speed as it wants, so long as no matter on the event horizon locally exceeds c. Except for the angular momentum limit.

I've not studied Relativity in too much depth, but this gives me a context to understand something I've always had a problem with re: the idea that the speed of light is a hard limit. Thanks!

There's still an issue here with exceeding the speed of light and causality.

If you exceed the speed of light with respect to any external observer, then that observer will see you arrive at your destination before you left. "Seeing" here implying that some information from you can reach them. According to that observer, effect will precede cause and either causality is broken, or the relativity principle is broken, and either way Relativity is broken.

In this sense, the speed of light is a global limit.

However there's only a causality problem if there's a potential for interaction. So for instance universal expansion implies that extremely distant objects are moving away from us faster than light, but by the same token no information can reach us from them so there's no possibility of causality violation. Same with something within the event horizon moving faster than light with respect to an outside observer -- no information can escape, so no causality violation.

Hypothetical FTL "cheats" like wormholes where you move FTL and then can hypothetically interact with the rest of the universe run into this problem. It's unknown how the universe deals with it.

More on topic, the possibility of apparent light speed violation because of stretching of the underlying spacetime is something people usually forget about the possibility of. I wonder how fast the frames are getting dragged right near the event horizon, and percentage-wise how much that increases the apparent speed of light relative to our frame.

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

Same way the electron, as a point particle, has spin. So long as you're massive, nothing prevents your wave function from carrying angular momentum, and when a black hole has angular momentum, space is dragged along for the ride. Actually, any rotating mass does this (it's been measured around the Earth) but it's most extreme around a black hole.

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

I do not think spin means what you think it means. No disrespect intended, spin turns out to be a lot more mysterious and strange than people's intuition. One of the many strange conceptual problems that physics faced in the last century had to do with exactly this issue. Things like electrons can be shown to have angular momentum, and people pictured this as a little spinning ball. But people kept trying to measure the diameter of the electron, and came up with a value for an upper limit that would require the surface of the spinning electron at that diameter to be moving faster than the speed of light. So clearly it can't be spinning the way that we think of a ball as spinning.

And as it turns out, ALL elementary particles (in the standard model) are dimensionless points, as far as is known. In other words, there isn't anything BUT dimensionless points. All other non-elementary particles are simply clusters of those dimensionless points.

So how does a dimensionless point spin? No one knows what that means intuitively, but they do it, because we can measure the fact that they have angular momentum. The best way to think about it is that quantum spin is more of a symmetry property of objects, not some property that involves "motion", and it can behave differently than our intuitions about spin. For example, electrons have a spin of 1/2, which means they must rotate twice on their axis before they come back to their original wavefunction state.

Now not only is that really bizarre, since everything else in our experience comes back to the same orientation after 1 rotation, but it is even more bizarre since it also means that if YOU go AROUND the electron once, that is equivalent to it rotating once, and so if you walk around an electron and then look at it, it isn't the same, it has a negated wavefunction, and you have to walk around it twice before it looks the same as it did at the beginning. This is a mysterious feature of the way spacetime works, and we just have to accept it, and accept that spin is something trickier than we think it is.

I knew that electrons had "spin" but was under the impression that it wasn't spin like in a top but was an arbitrary physics name like "charm". I didn't realize that it was related to angular momentum. How do measure the angular momentum of an electron?

Even allowing naked singularities, no point on the event horizon would be able to exceed the speed of light, so depending on the radius, you could determine a maximum spin rate.

The event horizon isn't a physical object. The speed of light limit is local, so the space there can move as fast as it likes, dragging matter around from the perspective of an external observer at as large an apparent speed as it wants, so long as no matter on the event horizon locally exceeds c. Except for the angular momentum limit.

Right, angular momentum needs to be conserved from every frame of reference. Remember, from the frame of reference of a particle falling into the black hole there IS no distinct event horizon either, each view of the situation is radically different, so different that the 2 cease to communicate (apparent difference in velocity reaches C from both frames). Exactly what frames are dragging and where the angular momentum is are also entirely relative. Complete solutions for what happens at the singularity itself of course don't exist.

Previous observations estimated that the black hole in the Great Barred Spiral Galaxy is spinning nearly as fast as possible; whether other black holes will have similar properties remains an open question.

Quick question from the ignorant: what does "nearly as fast as possible" mean? What's the limit on black hole rotational speed?

Well, in theory, thought the center can spin very fast, at a point, instead of spinning the matter near it, once that matter starts approaching light speed velocities itself, that matter can itself exert a "resistive" force on the spinning. Essentially, since the matter CAN'T go faster, spinning faster is thus impossible as doing so would create coinditions otherwise not possible, thus the boyancy of tyhat system as more matter is introduces and the mass of the spinning plate increases, the black hose stops accelerating it's own spin.

So inferring that your black hole is only half the solar system, this would put it below 1% the density of the sun...

Perhaps you should stick to telling us how many elephants the black hole weighs and how many 747s in diameter it is across. /troll

The size of the "black hole" is not the same as the size of the point mass at it's center. The "black hole" is the area insude the event horizon, but only a tiny fraction of that space is mass. In fact, the mass contained inside the singularity is compressed to a ball somewhat the size of a common sun but weighing several billion times as much. This creates an event horison several light days across, but that is not the size of the object in it creating that phenomenon.

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

I do not think spin means what you think it means. No disrespect intended, spin turns out to be a lot more mysterious and strange than people's intuition. One of the many strange conceptual problems that physics faced in the last century had to do with exactly this issue. Things like electrons can be shown to have angular momentum, and people pictured this as a little spinning ball. But people kept trying to measure the diameter of the electron, and came up with a value for an upper limit that would require the surface of the spinning electron at that diameter to be moving faster than the speed of light. So clearly it can't be spinning the way that we think of a ball as spinning.

And as it turns out, ALL elementary particles (in the standard model) are dimensionless points, as far as is known. In other words, there isn't anything BUT dimensionless points. All other non-elementary particles are simply clusters of those dimensionless points.

So how does a dimensionless point spin? No one knows what that means intuitively, but they do it, because we can measure the fact that they have angular momentum. The best way to think about it is that quantum spin is more of a symmetry property of objects, not some property that involves "motion", and it can behave differently than our intuitions about spin. For example, electrons have a spin of 1/2, which means they must rotate twice on their axis before they come back to their original wavefunction state.

Now not only is that really bizarre, since everything else in our experience comes back to the same orientation after 1 rotation, but it is even more bizarre since it also means that if YOU go AROUND the electron once, that is equivalent to it rotating once, and so if you walk around an electron and then look at it, it isn't the same, it has a negated wavefunction, and you have to walk around it twice before it looks the same as it did at the beginning. This is a mysterious feature of the way spacetime works, and we just have to accept it, and accept that spin is something trickier than we think it is.

Correct. The problem is, people think of an electron as a ball at all, when in reality, it's a collection of smaller parts orbiting each other, just like electrons orbit protons and neutrons. The idea is that the pattern of those orbits creates a "wobble" that has angular momentum. The time is 1/2 because the parts do not orbit in a plane around a center, but through and back again, each part moving in reaction to the others moving, and it takes 2 "spins" to reset all the parts to an originating (or equal in some other way) state. In fact, it's likely each of those parts has their own angular movement internally, and they're likely made of even yet smaller bits we're just starting to comprehend.

Want to really blow people's minds? It's possible that inside black holes, matter exists at negative kelvin temperatures..... a potential and kinetic energy so high, additional energy cannot be added, and in such a state, particles that would otherwise attract (and in fact cannot because they create negative pressure), would instead collapse in upon themselves instead or repel each other. This may be the fundamental force behind black hole matter/energy ejection phenomenon, and how the universe is expanding although gravity should be making it contract.

So inferring that your black hole is only half the solar system, this would put it below 1% the density of the sun...

Perhaps you should stick to telling us how many elephants the black hole weighs and how many 747s in diameter it is across. /troll

The size of the "black hole" is not the same as the size of the point mass at it's center. The "black hole" is the area insude the event horizon, but only a tiny fraction of that space is mass. In fact, the mass contained inside the singularity is compressed to a ball somewhat the size of a common sun but weighing several billion times as much. This creates an event horison several light days across, but that is not the size of the object in it creating that phenomenon.

No. The singularity at the center of a black hole is a dimensionless point, as far as is known.

I still don't get it. I thought a black hole was a singularity. A point mass. That would infer no diameter. How can something with no diameter have a spin. It seems there would be no way to tell where the "start" of a rotation is.

I do not think spin means what you think it means. No disrespect intended, spin turns out to be a lot more mysterious and strange than people's intuition. One of the many strange conceptual problems that physics faced in the last century had to do with exactly this issue. Things like electrons can be shown to have angular momentum, and people pictured this as a little spinning ball. But people kept trying to measure the diameter of the electron, and came up with a value for an upper limit that would require the surface of the spinning electron at that diameter to be moving faster than the speed of light. So clearly it can't be spinning the way that we think of a ball as spinning.

And as it turns out, ALL elementary particles (in the standard model) are dimensionless points, as far as is known. In other words, there isn't anything BUT dimensionless points. All other non-elementary particles are simply clusters of those dimensionless points.

So how does a dimensionless point spin? No one knows what that means intuitively, but they do it, because we can measure the fact that they have angular momentum. The best way to think about it is that quantum spin is more of a symmetry property of objects, not some property that involves "motion", and it can behave differently than our intuitions about spin. For example, electrons have a spin of 1/2, which means they must rotate twice on their axis before they come back to their original wavefunction state.

Now not only is that really bizarre, since everything else in our experience comes back to the same orientation after 1 rotation, but it is even more bizarre since it also means that if YOU go AROUND the electron once, that is equivalent to it rotating once, and so if you walk around an electron and then look at it, it isn't the same, it has a negated wavefunction, and you have to walk around it twice before it looks the same as it did at the beginning. This is a mysterious feature of the way spacetime works, and we just have to accept it, and accept that spin is something trickier than we think it is.

Correct. The problem is, people think of an electron as a ball at all, when in reality, it's a collection of smaller parts orbiting each other, just like electrons orbit protons and neutrons. The idea is that the pattern of those orbits creates a "wobble" that has angular momentum. The time is 1/2 because the parts do not orbit in a plane around a center, but through and back again, each part moving in reaction to the others moving, and it takes 2 "spins" to reset all the parts to an originating (or equal in some other way) state. In fact, it's likely each of those parts has their own angular movement internally, and they're likely made of even yet smaller bits we're just starting to comprehend.

No. The electron is not a collection of smaller parts. It is an elementary particle that cannot be decomposed into parts, and is a dimensionless point, in the standard model.